The present invention generally relates to methods for analyzing organosulfur compounds and organosulfur compound-containing substances, and, more particularly, to methods for analyzing organosulfur compounds and organosulfur compound-containing substances using optical measurement techniques, optionally in the presence of one or more interfering compounds.
Hydrogen sulfide (H2S) is a colorless and exceedingly poisonous gas that, in small quantities, has a foul odor of rotten eggs. In larger quantities, hydrogen sulfide dulls the olfactory senses and is essentially odorless. The latter represents an extremely hazardous situation, as serious injury or death may occur without one even being aware that they are being exposed to a dangerous substance. Common sources of hydrogen sulfide in the environment include, for example, metabolic waste products of anaerobic bacteria, volcanic eruptions, and natural gas emissions.
In the environment, organosulfur compounds frequently occur in concert with hydrogen sulfide. Many organosulfur compounds, particularly mercaptans (also known as thiols), also have a foul odor that is not greatly different than that of small quantities of hydrogen sulfide. Although some mercaptans and other organosulfur compounds may be toxic, they are generally much less so than hydrogen sulfide. As used herein, the term “mercaptan” refers to compounds having the formula RSH, where R comprises a carbon-containing group. Other organosulfur compounds include, for example, disulfides, sulfoxides, sulfones, sulfate esters, sulfonic acids, sulfonate esters, sulfinic acids, sulfinate esters, thiophenes, condensed thiophenes, combinations thereof, and the like. Hydrogen sulfide and mercaptans, in particular, can oftentimes be difficult to distinguish from one another due to their similar chemical and spectroscopic properties.
Once the presence of hydrogen sulfide and/or mercaptans has been established in a substance, profound implications can arise regarding the manner in which the substance is handled. In the health and safety regime, knowing whether an odiferous substance contains hydrogen sulfide, a mercaptan, or both, and the quantities of each can dictate appropriate handling techniques, disposal protocols, and any required personal protective equipment. From an operational standpoint, the acidity of hydrogen sulfide and mercaptans needs to be accounted for in engineering and process design due to the propensity of these compounds to promote corrosion and/or scaling on surfaces contacting the substance.
Despite the significant benefits that can be realized by analyzing for mercaptans and/or hydrogen sulfide in a substance, current techniques for doing so may be limited in their capabilities, particularly when both mercaptans and hydrogen sulfide are present. Non-spectroscopic chemical analyses may be complicated by difficult sample acquisition and/or preparation techniques and long analytical turn-around times, for example. Due to the similar chemical and spectral properties of hydrogen sulfide and mercaptans, spectroscopic analyses of hydrogen sulfide in the presence of mercaptans, or vice versa, may be complicated by significant spectral overlap, thereby making it difficult to distinguish between these compounds. Other spectral interferents such as carbon dioxide, for example, may also be problematic during spectroscopic analyses of hydrogen sulfide and/or mercaptans.
As noted above, the presence of hydrogen sulfide and/or mercaptans can have profound operational impacts in determining appropriate engineering solutions and process controls for an application. One industry in which hydrogen sulfide and/or mercaptans are frequently encountered is in the oilfield industry, where hydrogen sulfide and/or mercaptans are sometimes found downhole within a subterranean formation. Consequences of having mercaptans and/or hydrogen sulfide present during an oilfield operation may include corrosion of metal goods (e.g., tools, pipe, and the like) placed in the formation, formation of scale damage in the formation, or any combination thereof. In addition, the presence of excessive quantities of either type of compound, particularly hydrogen sulfide, may represent a health hazard for operational personnel. Depending on the quantity and location of hydrogen sulfide and/or mercaptans in the subterranean formation, different engineering solutions or process solutions may be needed to address their presence. For example, depending on the quantities of hydrogen sulfide and/or mercaptans present, it may sometimes be desirable to apply a corrosion-resistant coating to metal goods used downhole. In other cases, it may be more desirable to remediate the hydrogen sulfide and/or mercaptans directly, or, if possible, the source leading to their production.
Despite the advantages of analyzing for hydrogen sulfide and/or mercaptans during subterranean operations, state of the art techniques for doing so are surprisingly limited. Of the few techniques that are available, they may require difficult sampling, sometimes produce inaccurate results, and be too slow for real-time or near real-time process control. One technique that is commonly used for analyzing hydrogen sulfide and/or mercaptans in subterranean formations involves collection of a downhole fluid sample in a pressurized container for subsequent laboratory analysis. However, this technique can be prone to underestimation of the true quantities of these compounds due to their reaction with the container used for collection. Further, as noted previously, it may be difficult to analyze for these compounds in the presence of one another or in the presence of other interferents. Another technique that may be used to indirectly analyze for hydrogen sulfide and/or mercaptans in a subterranean formation involves a downhole metallic coupon test. Such tests, which measure corrosion, are slow, unable to distinguish between hydrogen sulfide and mercaptans, and of limited ability to determine the locale of hydrogen sulfide and/or mercaptans in the subterranean formation.
In addition to detecting and analyzing for hydrogen sulfide and/or mercaptans while conducting downhole operations, it can also be desirable to analyze for these substances once a produced fluid (e.g., oil or natural gas) has been sent for refining. If excessive quantities of one or both of these compounds are present, additional refining steps or alterations to the refining process may be needed to produce a refined fluid product having desired qualities. For example, hydrogen sulfide and mercaptans may poison catalysts used in conjunction with the refining process, and it may be desirable to decrease amounts of these compounds to levels that are more satisfactory to conduct the refining. Likewise, high levels of benzothiophenes in a produced fluid, for example, may lead to a refined product having sulfur levels that are too high to meet current environmental standards. If hydrogen sulfide, mercaptans, and/or other organosulfur compound levels in a subterranean formation are too excessive, it may not be economically feasible to refine a fluid produced therefrom, even if appropriate engineering solutions and process controls are undertaken during drilling and production. Thus, a decision may sometimes be made not to produce a subterranean formation based upon the formation's hydrogen sulfide and/or mercaptans content. In the event that higher than expected hydrogen sulfide and/or mercaptan levels are encountered once a wellbore has been placed into production, costly re-engineering and redesign may be needed to safely and effectively produce the formation. In view of the foregoing, the ability to accurately and rapidly determine the hydrogen sulfide and/or mercaptans content of a subterranean formation may be desirable.